1. Field of the Invention
This invention relates to a process and apparatus for the manufacture of a tube composed of concentric inner and outer tubes. This invention is particularly directed to the production of a double-walled synthetic plastic tube which has an outer tube which is annularly or helically corrugated. This invention is particularly directed to a process which will insure that the inner tube is properly disposed with respect to the outer tube, particularly when the outer tube has helical corrugations so that the inner tube uniformly touches the nadir of the corrugations of the outer tube.
2. DISCUSSION OF THE PRIOR ART
According to German patent specification 1,704,718 it is known to produce synthetic plastic tubes having an annularly or helically corrugated outer wall and a substantially smooth inner wall by extruding two concentric tubes in one operation. The outer tube is extruded from an annular extrusion die into a mold cavity formed by divided molds having transverse internal grooves and forming two cooperating endless recirculating trains of mold halves which close to complete the mold cavity. The tube is drawn into the transverse grooves by vacuum pressure. The inner tube is extruded from a die extending into the interior of the molds and brought into close contact with the fully formed outer tube.
This method permits the construction of double-walled tubes but where the tubes have a smaller cross section and are to be produced at a high production rate, difficulties arise because of pressure differentials within the region between the inner tube and the outer tube. These pressure differentials cause the inner tube to be drawn into the corrugations of the outer tube or to form inwardly projecting bulges between the internal press of the corrugations. Such tubes lack the desired smooth internal surface which instead is slightly angular. This angular construction considerably increases the drag when liquids or gases are conducted through such tubes.
According to U.S. Pat. No. 2,280,480 it is known to produce double-walled tubes by dividing the stream of plastic to be extruded from the extruder into two component streams which emerge from two annular extrusion dies arranged in tandem. It is also known to introduce compressed air between these annular dies and into the interior of the inner tube for effecting the desired forming operation. However, control of the two synthetic plastic streams and of the relative pressures thereof with respect to the compressed air presents considerable difficulties in commercial operations.
It has, therefore, become an object of the present invention to overcome these difficulties and to provide a double-walled tube wherein the inner wall is substantially smooth and is neither drawn into the corrugations of the outer tube nor caused to form inwardly projecting annular bulges. It is another object of this invention, therefore, to provide a double-walled tube wherein the outer wall has helical corrugations and the inner wall is smooth further characterized in that the inner wall touches the outer wall only at the nadir of its corrugations.
It has long been recognized that problems are encountered in the formation of double-walled tubes especially those having helical corrugations which difficulties are caused by certain phenomena experienced during the cooling of the freshly extruded molten synthetic plastic. Thus, it was found that when the inner tube which is still plastically formable is cooled that the pressure in the space between the tubes is different from the pressure within the inner tube itself. The resultant pressure differences cause the inner wall to be additionally deformed. In the case of tubes having a helically corrugated outer wall, the intermediate space between the tubes communicates with the ambient atmosphere. However, the communicating channel is of considerable length and can be as long as 28 meters per meter of tube in a tube having an internal diameter of 6 cm. and 150 corrugations per meter. In other words, when 10 meters of each tube are being extruded, the total length of this channel is already 280 meters.
In view of the length of this communicating channel, pressure fluctuations inside the corrugations cannot be compensated quickly enough by the ambient pressure and, depending upon the rate at which the air in these corrugations is heated or cooled, considerable gauge or vacuum pressures may actually rise. If the tube is provided with a self-contained annular corrugation, a compensation of pressure fluctuations by the ambient pressure is virtually impossible.
A smooth inside wall can, therefore, be obtained only if the production process can be suitably controlled to insure that at least in the zone where the inner tube is still in the plastically formable state and has not yet fully set, the pressure in the space between the two tubes is the same as the pressure inside the inner tube.